fix: Notification page breakages (#5236)

- Remove the cascading foreign key indexes
- Add migration to clean up existing objects

fixes: #4285

app/models/data_import.rb

@@ -16,10 +16,6 @@
 #
 #  index_data_imports_on_account_id  (account_id)
 #
-# Foreign Keys
-#
-#  fk_rails_...  (account_id => accounts.id) ON DELETE => cascade
-#
 class DataImport < ApplicationRecord
   belongs_to :account
   validates :data_type, inclusion: { in: ['contacts'], message: I18n.t('errors.data_import.data_type.invalid') }